At present the application of digital multisensor detectors in medicine, astronomy and the other areas is continuously extended [Howell S. B. Handbook of CCD Astronomy. Cambridge University Press, 2000; Gino M. Noise, Noise, Noise; S. I. Miroshnichenko, A. A. Nevgasimyj Theory and technique of multisensor digital X-ray receivers. Biotechnosphere, No. Apr. 10, 2010; B. Yane Digital processing of images, M., Technosphera, 2007].
The image in multisensor detectors can be formed by several CCD or CMOS sensors. Each sensor of multisensor detector, in its turn, can consist of smaller sensors with their own signal conversion path. Gain characteristics of each sensor of such detector, that determine output values of signal intensity of the formed image will differ due to technologic and other reasons. The discrepancy of gain characteristics result in common inhomogeneity of response (inhomogeneity of output image) of detector, in occurrence of discontinuous changes (so called stitches) at the joints of sensors in the resulting digital image. Therefore, in actual practice, when using multisensory detectors, the task arises to stitch the gain characteristics of sensors, constituting the same detector. For the purpose of image acquisition, homogeneous in response and without any stitches at the sensor joints, the procedure of calibration shall be applied which could stitch all the characteristics to the set one. In addition to combining, in order to perform calibration of the flat field, the task of gain characteristics linearization shall be settled.
There are several approaches to settle the task of stitching characteristics. In case of sensors linear responses the standard approach is so called two-point calibration. The methods of stitching and linearization of their characteristics at highly nonlinear characteristics of CMOS-sensors are known. For example, the method of stitching and linearization pixel by pixel of CMOS sensors characteristics is described in the source [Rad-icon Imaging Corp. AN08: Polynomial Gain Correction for RadEye Sensors, www.rad-icon.com/pdf/Radicon AN08.pdf)], wherein:
Using the source of light field with equal distribution of intensity along the detector field-of-view (FOV), two calibration images are received at two levels of input signal, the first level of input signal being selected twice as little as the next one;
Parameters of quadratic dependence simulating the sensor response are determined and using the calibration images received the correction function is set up which linearizes and stitches pixel characteristics of CMOS-sensor.
The other method of stitching and linearization of CMOS-sensors pixels is described in the source [Liji C., Jorg P. A Practical Non-linear Gain Correction Method for High-resolution CMOS Imaging Detectors], wherein:
Three calibration images at three various levels of output signal are received using light field with equal distribution of intensity along the detector field-of-view;
The responses of sensor pixels are simulated by piecewise quadratic plain dependence of three segments;
Parameters of model dependence are determined and using the calibration images received the correction function is set up which linearizes and stitches pixel characteristics of CMOS-sensor.
The nearest to the inventive engineering solution is the method described in the source [Kodak. Multiple Output Sensors Seams Correction. Application Note, 2009], wherein the sensor-by-sensor linearization and stitching of the detector characteristics is performed.
Within this method:
The series of N calibration images is received at increasing values of radiation intensity using the light field source with equal distribution of intensity along the detector field-of-view;
The sensor responses are measured by way of LUT functions describing dependence of output signal on the values of the input signal,
The measured sensor responses are linearized and stitched to the one response arbitrarily selected among these linearized responses.
In all the methods of linearization and stitching of gain characteristics listed above, including the method [Kodak. Multiple Output Sensors Seams Correction. Application Note, 2009], the light field source is used with equal distribution of intensity along the detector filed-of-view. However, in some cases it is inconvenient or essentially impossible to use such source. The inconvenience is connected with difficulty of its generation. The impossibility to use light field source with equal distribution of intensity along the detector filed-of-view can be caused, for example, by designer's availability of detector with already integrated scintillation screen converting X-ray radiation to light. In the latter instance, only non-planar X-ray field turns out to be available to stitch and linearize gain characteristics of sensors of multisensory detector causing non-equal distribution of intensity along the detector filed-of-view (non-equal irradiance (light) illumination).
The present invention object is the development of method of stitching and linearization of detector gain characteristics under the conditions of non-equal irradiance.